1. Field of the Invention
The present invention relates to a radio tag, and in particular to a noncontact type radio tag communicating with an RFID reader/writer.
2. Description of the Related Art
In recent years, an RFID (Radio Frequency IDentification) system identifying/managing objects by unique information stored in an IC chip has been actively developed. In the RFID system, an RFID reader/writer transmits a high-frequency electric wave signal. A radio tag provided with the IC chip having stored therein the unique information receives the electric wave signal, and then transmits the unique information to the RFID reader/writer.
For example, by attaching radio tags to commercial products such as books and clothes, it is made possible to read the unique information of the radio tag attached to the commercial products by using the RFID reader/writer, or reversely write the unique information in the radio tag.
The radio tag is generally composed of the IC chip and an antenna. When a high-frequency signal is received by the antenna, a rectifier portion embedded in the IC chip converts the high-frequency signal into a DC component on the order of 3 V, for example, so that the signal can be processed by the electric power thereof and further transmitted through the antenna.
As for the frequency of the signal, not only 13.56 MHz but also higher frequencies such as 900 MHz and 2.45 GHz have been used recently.
FIG. 5 shows an example of an RFID tag as a generally used radio tag. An RFID tag 300 in FIG. 5 is composed of a dipole antenna 11 of a plane circuit type having a length of λ/2 and an IC chip 6.
Depending on the shape of the dipole antenna 11 and the chip power consumption in the IC chip 6, if the transmission signal power from the RFID reader/writer (not shown) or the radio tag is on the order of 1 W, the RFID reader/writer and the radio tag have a communicable distance on the order of 1 m.
As for other prior art radio tags, there is one in which a semiconductor chip storing information of an object to be identified, a first antenna capable of receiving and transmitting the information of the semiconductor chip between a receiving/transmitting apparatus provided externally, and a second antenna operable by receiving an electric wave of a frequency different from that of the first antenna are integrated (see e.g. patent document 1).
This has integrated an identification tag and an antitheft tag, for example, to improve convenience in handling.
Also, there is another in which two radio tags whose axial directions of cylindrical antenna coils are arranged almost orthogonal with each other and are mutually fixed by a fixing means, thereby relieving directional restriction of transmission/reception sensitivity when the axial directions of the cylindrical antenna coils are arranged almost parallel to the affixing surface of the affixing member (see e.g. patent document 2).
This has enabled a signal transmission/reception by avoiding influences of a conductive member by utilizing a leaked electric wave even when an object to which a radio tag is attached is a conductive member such as a metal having influence on communication and power transfer of a radio tag by generating magnetic flux in the opposite direction for attenuating the original magnetic flux by an eddy current.
Moreover, there is a radio tag capable of transmitting/receiving a signal even if the radio tag is embedded in a conductive member such as a metal (see e.g. patent document 3).
(Patent Document 1) Japanese Patent Application Laid-open No.2000-339422
(Patent Document 2) Japanese Patent Application Laid-open No.2002-183695
(Patent Document 3) Japanese Patent Application Laid-open No.2003-85515
The above-mentioned patent documents 2 and 3 resolve a problem in a case where an object to which the radio tag is attached is a conductive member such as a metal. Specifically, the above-mentioned patent document 3 discloses a radio tag capable of transmitting/receiving a signal even if it is embedded in a metal or the like.
However, it has been known that a radio tag using a patch antenna may be used for merely attaching a tag to a surface of a metal or the like.
FIG. 6 shows an RFID tag 400 using a patch antenna 2 as such an example. The RFID tag 400 has the square patch antenna 2 whose side has a length of λ/2 on the top surface of a dielectric substrate 1 as shown in FIG. 6. Also, on the undersurface thereof, a ground 8 is formed all over the surface and functions as a ground of the patch antenna 2.
The patch antenna 2 is connected to the IC chip 6 through a microstrip line 3 provided on the same surface, and is fed with electric power from the IC chip 6 through the microstrip line 3. Also, the IC chip 6 is connected to the ground 8 on the undersurface through a through hole 7.
For the RFID tag 400 in FIG. 6, if the surface to which the patch antenna 2 is not attached, i.e. the ground 8 of the undersurface is attached to a metallic object, the ground 8 and the metallic object assume the same electric potential, so that the electric potential of the patch antenna 2 itself does not change and therefore its input impedance does not change. Thus, the RFID tag 400 can be attached to the metallic object to be used.
FIGS. 7A and 7B schematically show how an electric wave is received in this case. As shown in FIG. 7A, when the surface of the ground 8 is attached to a metallic object 20, a signal S1 arriving at the surface of the patch antenna 2 can be received by the patch antenna 2. As a matter of course, since a signal S2 arriving at the surface of the ground 8 is intercepted and reflected by the metallic object 20, it cannot be received.
Contrarily, as shown in FIG. 7B, when the surface of the patch antenna 2 is attached to the metallic object 20, the patch antenna 2 itself is connected to the electric potential of the metallic object 20, so that not only its input impedance changes, but also the signal S1 is reflected since the ground 8 is metallic. Namely, when the surface of the patch antenna 2 is attached to the metallic object 20, neither the signal S2 arriving at the side of the patch antenna 2 nor the signal S1 arriving at the surface of the ground 8 can be received.
Therefore, in case of the RFID tag 400, it is required that the side of the ground 8 is attached to the metallic object 20 without fail as shown in FIG. 7A and that transmission and reception of an electric wave are enabled only in the direction of the top surface where the patch antenna 2 is attached (direction of signal S1 in FIG. 7A).
Also, when the object to which the RFID tag 400 is attached is nonmetallic, the patch antenna 2 or the ground 8 may be attached to the object. However in this case, the electric wave from the direction incident on the ground 8 is reflected by the metallic ground 8.
FIGS. 7C and 7D schematically show how a signal is received in cases where the RFID tag 400 is attached so that the ground 8 and the patch antenna 2 may respectively touch a nonmetallic object 30.
In either case, although both of the signals S1 and S2 have reached the RFID tag 400, when the ground 8 is attached to the nonmetallic object 30 as shown in FIG. 7C, the signal S1 shown can be received by the patch antenna 2 but the signal S2 is reflected by the ground 8 and cannot be received. Contrarily, when the patch antenna 2 is attached to the nonmetallic object 30 as shown in FIG. 7D, the signal S2 shown can be received by the patch antenna 2 but the signal S1 is reflected by the ground 8 and cannot be received.
In contrast, the above-mentioned RFID tag 300 shown in FIG. 5 has a problem that when it is attached to a metallic object the input impedance of the dipole antenna 11 is largely changed, so that the communication distance is extremely shortened or the communication is completely disabled. However, if the object to be attached is not metallic, the RFID tag 300 has a directivity in all directions except null directions A and B.
Therefore, the RFID tag 300 is more suitable than the RFID tag 400 when it is attached to a nonmetallic object.
Supposing a case where metallic objects and nonmetallic objects coexist as the objects to which the radio tags are attached, if it is not desired to limit the directivity of the radio tags to be attached to the nonmetallic objects to only one direction, the preparation of two types of radio tags, for example, the RFID tag 400 in FIG. 6 and the RFID tag 300 in FIG. 5, respectively for the metallic objects and the nonmetallic objects is required.
In this case, it is required to identify whether or not the object to be attached is metallic at the time of attachment. If the RFID tag 300 for the nonmetallic object is attached to the metallic object by mistake, the communication distance will be extremely shortened or the communication will be completely disabled as mentioned above.
Contrarily, if the RFID tag 400 for the metallic object is attached to the nonmetallic object, the transmission/reception of the electric wave for only one surface can be performed as shown in FIGS. 7C and 7D regardless of whether the attached surface is the patch antenna 2 or the ground 8, so that there is a problem that the directivity in overall directions (except null direction) expected for the RFID tag 300 for the nonmetallic object which should have been originally attached cannot be obtained.
Also, it is possible to use e.g. the RFID tag 400 of FIG. 6 if one type of radio tag is commonly used for the metallic object and the nonmetallic object. However, there is a problem that in this case, the directivity of the RFID tag 400 when it is attached to the nonmetallic object is relatively weak compared to that of the RFID tag 300 of FIG. 5 as mentioned above.